The present invention relates to the manufacture of electronics apparatus, and more particularly concerns the construction of such apparatus so as to facilitate rework when defective components are found.
Miniaturized electronics equipment for portable computers, cameras, automobiles, aircraft, space vehicles, and many other applications frequently employ a number of unpackaged semiconductor chips or dice mounted on a carrier having a printed wiring pattern for connecting the chips to each other and to other components. This is commonly known as "direct chip attach" (DCA). The carrier may be a thin rigid sheet of fiberglass epoxy, ceramic, coated metal, or other material; it may also be a flexible sheet of polyamide or other suitable material. Usually the rear faces of the chips are affixed to the carrier with an adhesive, and the chip contacts on the front faces are coupled to the carrier wiring by means of a conventional technique called wire-bonding.
The unpackaged diced chips employed in such applications are difficult to test as individual components; they are small, and placing test probes at the contact points requires operators using microscopes. At the same time, the reliability of unpackaged chips is relatively low: less than 90-95%, even after wafer testing by the supplier. Thus, an assembly of 5-7 chips on a carrier may have an overall manufacturing yield in the range of 70% or even less.
The substantial cost and relatively high fallout of many devices built with DCA technology mandate an inexpensive way to rework assembled devices which have failed a final manufacturing test. However, the adhesives employed to attach chips to carriers commonly do not allow the removal of defective chips or other components from the carrier. This prevents the removal of individual components and the substitution of replacements.
Affixing chips directly to a carrier with solder facilitates easy removal by applying local heat through the chip to melt the solder. An advantage of this technique is that the solder layer for this purpose is commonly required for the mounting of other components on the carrier. That is, attaching the chips requires no process step beyond those already required for other reasons. However, the rear faces of the chips must be metallized so as to wet the solder; this is prohibitively expensive in many cases. Moreover, the thermal expansion coefficients of the chips and the carrier may differ from each other sufficiently to fracture the joint or even the chip.
Some chips are mounted face down directly on a carrier, so that solder balls or columns between the chip contacts and the carrier wiring for electrical contact also hold the chip in place mechanically. This technology also allows removal by local heat, but it is generally not cost-effective except for very high function multichip modules on precision ceramic substrates.
Some workers in this art have employed adhesives which can be melted or softened at elevated temperatures. These adhesive materials do not separate cleanly when a defective chip is removed. They remain partially or entirely on the carrier, and must be removed by prohibitively delicate, expensive, and slow operations such as mechanical grinding and chemical treatment.